Lung function testing in neonates and infants with pulmonary disease is difficult and not routinely done. One such disease, bronchopulmonary dysplasia (BPD) is a major cause of morbidity in former preterm infants. It results in multiple abnormalities of lung mechanics, e.g., decreased lung compliance and increased airway resistance and work of breathing. Altered airways resistance leads to increased negative intrapleural pressure on inspiration which can affect chest wall motion, especially in young preterm infants with highly compliant chest walls. This can alter the rib cage and abdomen's normally synchronous motion, in which both compartments move in and out nearly simultaneously during breathing, and cause asynchrony of one compartment relative to the other. The extreme degree of this asynchrony is paradoxic breathing, in which inward motion of the rib cage occurs during outward motion of the abdomen. The broad objective of this proposal is to assess the interrelationship of chest wall asynchrony and underlying lung mechanics, and to determine if measures of thoraco-abdominal asynchrony provide a useful non-invasive method of assessing lung function and its response to therapy in infants with pulmonary disease. Lung mechanics will be measured during tidal breathing from measurements of esophageal pressure and flow at the mouth. Rib cage/abdomen asynchrony will be assessed with the respiratory inductive plethysmograph. We will answer the following questions: 1) Do abnormal pulmonary mechanics in infants with obstructive airways disease result in asynchronous rib cage/abdominal wall motion? 2) Does the improvement in lung mechanics in infants with obstructed airway treated with bronchodilators lead to more normal (i.e., synchronous) chest wall motion? 3) Conversely, does resistive loaded breathing in normal infants lead to more asynchronous chest wall motion? 4) How do gestational and postnatal age affect the relationship between lung mechanics and thoraco-abdominal asynchrony?